apacheGH-41089: [C++] Clean up remaining tasks related to half float …

…casts (apache#41084)

### Rationale for this change

Address remaining tasks brought up post-merge in this PR: apache#40067

### What changes are included in this PR?

Adds a couple of tests and cleans up cruft on another test.

### Are these changes tested?

Yes, as these are test-only related changes.

### Are there any user-facing changes?

No.

* GitHub Issue: apache#41089 

Authored-by: Clif Houck <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>

cpp/src/arrow/ipc/json_simple_test.cc

@@ -189,21 +189,6 @@ class TestIntegers : public ::testing::Test {
 
 TYPED_TEST_SUITE_P(TestIntegers);
 
-template <typename DataType>
-std::vector<typename DataType::c_type> TestIntegersMutateIfNeeded(
-    std::vector<typename DataType::c_type> data) {
-  return data;
-}
-
-// TODO: This works, but is it the right way to do this?
-template <>
-std::vector<HalfFloatType::c_type> TestIntegersMutateIfNeeded<HalfFloatType>(
-    std::vector<HalfFloatType::c_type> data) {
-  std::for_each(data.begin(), data.end(),
-                [](HalfFloatType::c_type& value) { value = Float16(value).bits(); });
-  return data;
-}
-
 TYPED_TEST_P(TestIntegers, Basics) {
   using T = TypeParam;
   using c_type = typename T::c_type;
@@ -212,17 +197,16 @@ TYPED_TEST_P(TestIntegers, Basics) {
   auto type = this->type();
 
   AssertJSONArray<T>(type, "[]", {});
-  AssertJSONArray<T>(type, "[4, 0, 5]", TestIntegersMutateIfNeeded<T>({4, 0, 5}));
-  AssertJSONArray<T>(type, "[4, null, 5]", {true, false, true},
-                     TestIntegersMutateIfNeeded<T>({4, 0, 5}));
+  AssertJSONArray<T>(type, "[4, 0, 5]", {4, 0, 5});
+  AssertJSONArray<T>(type, "[4, null, 5]", {true, false, true}, {4, 0, 5});
 
   // Test limits
   const auto min_val = std::numeric_limits<c_type>::min();
   const auto max_val = std::numeric_limits<c_type>::max();
   std::string json_string = JSONArray(0, 1, min_val);
-  AssertJSONArray<T>(type, json_string, TestIntegersMutateIfNeeded<T>({0, 1, min_val}));
+  AssertJSONArray<T>(type, json_string, {0, 1, min_val});
   json_string = JSONArray(0, 1, max_val);
-  AssertJSONArray<T>(type, json_string, TestIntegersMutateIfNeeded<T>({0, 1, max_val}));
+  AssertJSONArray<T>(type, json_string, {0, 1, max_val});
 }
 
 TYPED_TEST_P(TestIntegers, Errors) {
@@ -289,12 +273,6 @@ INSTANTIATE_TYPED_TEST_SUITE_P(TestUInt8, TestIntegers, UInt8Type);
 INSTANTIATE_TYPED_TEST_SUITE_P(TestUInt16, TestIntegers, UInt16Type);
 INSTANTIATE_TYPED_TEST_SUITE_P(TestUInt32, TestIntegers, UInt32Type);
 INSTANTIATE_TYPED_TEST_SUITE_P(TestUInt64, TestIntegers, UInt64Type);
-// FIXME: I understand that HalfFloatType is backed by a uint16_t, but does it
-// make sense to run this test over it?
-// The way ConvertNumber for HalfFloatType is currently written, it allows the
-// conversion of floating point notation to a half float, which causes failures
-// in this test, one example is asserting 0.0 cannot be parsed as a half float.
-// INSTANTIATE_TYPED_TEST_SUITE_P(TestHalfFloat, TestIntegers, HalfFloatType);
 
 template <typename T>
 class TestStrings : public ::testing::Test {